This may sound completely insane... and is nothing more than speculation, so take it for what little it is worth. I keep hearing they were very social, involved in little league and church and so on, yet they rarely had visitors. There WAS a visit from CPS a few months ago, and we've been told Isabel wrote on walls with crayons (that last will and testament she "scrawled" on the inside of the closet wall notwithstanding) added to Sergio specifically mentioning that everyone took showers... what if there was something about the condition of the inside of their home that either put them at risk if people found out (drugs, guns, etc.) or (as is more likely) would have embarrassed them (something like hoarding or poor housekeeping). I have seen some people with hideous houses whose children leave the house looking scrubbed and well dressed, not "neglected" or wearing stained and rumpled clothing. These are the kind of people who spend three days frantically cleaning before relatives come for a holiday, but who just can't throw anything away. In some cases, they are the ones who come outside onto their porches and close the door behind them when you show up unannounced. I suspect that if Social Services had gotten wind of that sort of thing they would have paid a visit to the family. It would have no bearing at all on Isabel disappearing, but may have been the catalyst for them to want to have a service come in and clean the house?
Just thinking out loud.